Field of the Invention
The present invention relates to the field of probe-based methods for scanning and imaging the surface of a material. In particular, the invention is directed to fast raster scan imaging of delicate samples under low load conditions.
Description of Related Art
Scanning probe microscopy (SPM) techniques rely on using a physical probe in close proximity with a sample surface to scan the sample surface while controlling interactions between the probe and the surface. An image of the sample surface can thus be obtained, typically in a raster scan of the sample. In the raster scan the probe-surface interaction is recorded as a function of position and images are produced as a two-dimensional grid of data points.
The lateral resolution achieved with SPM varies with the underlying technique: atomic resolution can be achieved in some cases. Use can be made of piezoelectric actuators to execute scanning motions with precision and accuracy, at a length scale up to atomic or better. The two main types of SPM are the scanning tunneling microscopy (STM) and the atomic force microscopy (AFM). In the following, acronyms STM/AFM may refer to either the microscopy technique or to the microscope itself.
In particular, an AFM is a device in which the topography of a sample is modified or sensed by a probe mounted on the end of a cantilever. As the sample is scanned, interactions between the probe and the sample surface cause pivotal deflections of the cantilever. The topography of the sample may thus be determined by detecting the deflection of the probe.
Furthermore, as explained on the WWW pages of the National Institute of Standards and Technology (NIST), see bfrl.nist.gov/nanoscience/BFRL_AFM.htm, in AFM, the probe usually includes a sharp tip (nominal tip radius on the order of 10 nm) located near the end of the cantilever beam. The tip is raster scanned across the sample surface using, for example, piezoelectric scanners. Changes in the tip-sample interaction are often monitored using an optical lever detection system, in which a laser beam is reflected off of the cantilever and onto a position-sensitive photodiode. When scanning, a particular operating parameter is maintained at a constant level, and images are generated through a feedback loop between the optical detection system and the piezoelectric scanners. For a scanning stylus atomic force microscope, the probe tip is scanned above a stationary sample, while in a scanning sample design, the sample is scanned below a fixed probe tip.
Applications of AFM and other types of SPM are growing rapidly and include biological materials (e.g., studying DNA structure), polymeric materials (e.g., studying morphology, mechanical response, and thermal transitions), and semiconductors (e.g., detecting defects). In particular, AFM can be utilized to evaluate the surface quality of products such as contact lenses, optical components and semiconductor wafers after various cleaning, etching, or other manufacturing processes.
Three main imaging modes are known which can be used to obtain topographic images: contact mode, non-contact mode, and intermittent contact or tapping mode.
In contact mode, the probe is dragged across the surface. A constant flexure, or bend in the cantilever is maintained, which corresponds to a displacement of the probe tip relative to the undeflected position of the cantilever. As the topography of the surface changes, the relative position of the tip with respect to the sample must be moved to maintain this constant deflection. The topography of the surface can thus be mapped using this feedback mechanism; it is assumed that the motion of the z-scanner corresponds to the sample topography. To minimize the amount of applied force used to scan the surface, low spring constant (e.g. k<1 N/m) probes are used. However, significant deformation and damage of soft samples (e.g., biological and polymeric materials) often occurs during contact mode imaging (especially in air due to the force to be applied to overcome surface contamination). The combination of the applied normal force, the lateral forces resulting from dragging the probe tip across the sample, and the small contact areas involved result in contact stresses that can damage the sample, the tip, or both.
To overcome this limitation, contact mode imaging can be performed within a liquid environment, which lowers the deformation and damage problems by allowing the use of lower contact forces. Yet, imaging samples in a liquid environment is not always possible or practical.
To reduce the damages inherent to contact mode imaging, the cantilever can be oscillated near its first (or fundamental) bending mode resonance frequency (e.g. on the order of 100 kHz) as the probe is raster scanned above the surface in either non-contact mode or tapping mode.
In non-contact mode, both the tip-sample separation and the oscillation amplitude are on the order of 1 nm to 10 nm, such that the tip oscillates just above the surface, essentially imaging the surface of the sample. The resonance frequency and amplitude of the oscillating probe decrease as the sample surface is approached due to long-range forces extending above the surface (e.g. Van der Waals). Either a constant amplitude or constant resonance frequency can be maintained through a feedback loop with the scanner and, just as in contact mode, the motion of the scanner is used to generate the surface image. To reduce the tendency for the tip to be pulled down to the surface by attractive forces, the cantilever spring constant is normally much higher compared to contact mode cantilevers. The combination of weak forces affecting feedback and large spring constants causes the non-contact AFM signal to be small, which leads to unstable feedback and requires slower scan speeds than either contact mode or tapping mode. Also, the lateral resolution in non-contact mode is limited by the tip-sample separation and is normally lower than that in either contact mode or tapping mode.
Tapping mode tends to be more applicable to general imaging in air, particularly for soft samples, as the resolution is similar to contact mode, whereas the forces applied to the sample are lower and less damaging. A main disadvantage of the tapping mode relative to contact mode is the slower scan speed.
In tapping mode, the cantilever oscillates close to its first (fundamental) bending mode resonance frequency, as in non-contact mode. However, the oscillation amplitude of the probe tip is much larger than for non-contact mode, often in the range of 20 nm to 200 nm, and the tip makes contact with the sample for a short duration in each oscillation cycle. As the tip approaches the sample, the tip-sample interactions alter the amplitude, resonance frequency, and phase angle of the oscillating cantilever. During scanning, the amplitude at the operating frequency is maintained at a constant level, called the set-point amplitude, by adjusting the relative position of the tip with respect to the sample. In general, the amplitude of oscillation during scanning is large enough such that the probe maintains enough energy for the tip to tap through and back out of the surface.
As said, one of the main disadvantages of scanned probes in general, and atomic force microscopy (AFM) in particular, is the relatively low scanning speed. There is a trade-off between scanning speed and wear reduction, especially when the aim is to reduce damage of the surface to be imaged by the probe. In general, the methods that achieve reduction of sample damage tend to slow down the scanning process. These methods mostly rely on avoiding sliding friction, using dynamic techniques, such as the tapping or non-contact modes described above. On the contrary, techniques such as contact mode AFM are potentially much faster, but they are less suitable for imaging delicate surfaces.